Method of making a fibrous fissionable member



United States Patent ()fiice 3,177,578 Patented Apr. 13, 1965 3,177,578METHOD @F MAKING A FIBROUS FISSIONABLE MEMBER Harold N. Barr, Baltimore,Md, assignor to Martin- Marietta Corporation, a corporation of MarylandNo Drawing. Filed Mar. 28, 1961, Ser. No. 98,760

. 4 Claims. (Cl. 29-474.3)

This invention relates to a novel method of manufacture and is acontinuation-in-part of my copending application Serial No. 723,095,filed Mar. 24, 1958, entitled Fissionable Member and Method of MakingSame, now abandoned. More particularly, it is concerned with themanufacture of a fibrous fissionable member which can be used in nuclearreactors.

In the reactor field, rods or tubes containing fissionable material maybe suitably spaced in a nuclear reactor for emission of energy which isabsorbed by a liquid or gas medium in contact with such members. Thedensity of the fissionable material in the rod or tubular member isimportant because at the high temperature levels of operation voidsbetween the fuel core material and the metal clad which normally encasesthis material gives rise to poor heat transfer properties causingpremature breakdown. in the manufacture of these members the aim is toeliminate void spaces betwen the core and the clad and to enhancethermal conductivity of the core to prevent formation of hot spots.

Present day fuel elements are for the most part formed of powdermaterials using conventional powder metallurgy methods. These techniquesare well established and include a cold press-sinter technique or a hotpressing technique each of which may be followed by a convent-ional hotworking operation. It is emphasized that such operations are availableto those skilled in the art only with regard to powder materials. Manyrestrictions are imposed upon techniques for providing fuel elementsusing powder materials in conjunction with cold press formingtechniques. Using the cold pressing method, it. is found that at thetemperatures of operation, separation of the binding agent, whether itbe metal or other material, from the ceramic or fissionable material,occurs. This results in a reduction of thermal conductivity and the lifeof the radioactive member is shorter than desired.

When such an element is made by hot pressing, the product is lessobjectionable than that obtained by the cold pressing technique.However, by the hot pressing technique the size and the thermalproperties of the member are limited and for that reason it oftenappears desirable to resort to other practices for its manufacture.

The present invention is concerned with a novel method of manufacturinga ceramic-metal fiber nuclear fuel element having exceptionalcharacteristics over those produced by convention techniques.

To date, attempts have been made a-t'manufacturing fuel elements using ametal fiber in lieu of metallic powder. Because of unique ditficultiesof manufacture, the methods for forming fibrous fuel elements have beenrestricted to hot pressing arrangements.

Particular restrictions present in cold pressing methods formanufacturing fibrous compacts include the necessity of providing lowdensity compacts because of the poor sinterability between theceramic-metal fiber combination. Presently, only hot pressing techniqueshave been found acceptable to attain high bulk density compacts. To thepresent time the cold pressing of fibrous elements such as UO -rnetalfiber has proved unsatisfactory. The metal fibers, being relativelyrigid, did not permit the thoria powder to contract uniformly. As aconsequence, specimens are distorted, have numerous cracks, and havebulk densities not much higher than in the green state.

Hot pressing has been employed to eliminate these features. With thistechnique, the fiber structure was compressed, forcing the oxide powderinto voids, thus developing very high bulk densities.

Although hot pressing techniques are workable with fibrous elements onlythose metallic materials which will not be affected by the temperatureat which the hot press ing will be accomplished may be utilized.Obviously, a multitude of low melting point materials as well as thosereactive with ceramics are precluded by a hot presing operation.

It is, therefore, an object of this invention to provide method ofproducing a ceramic powder-metal fibre fuel element using cold workingtechniques while still achieving a high fuel density with the element.

Another object of this invention is to provide a novel article ofmanufacture which is especially useful as a high temperature thermalconducting means.

Another object of this invention is to provide a novel method ofproducing a body from a ceramic powdermetal fibre mixture which hasespecially good properties in regard to thermal conductivity, heattransfer and resistance to thermal stress.

Another object of this invention is to provide a novel method ofproducing a fuel element having a core containing a high weightpercentage of fissionable material, and which possesses outstandingthermal properties.

Other objects and advantages of this invention will become apparent fromthe following description and explanation thereof.

In accordance with this invention, the article of manufacture comprisesa metallic cylindrical member having encased therein a core comprising aceramic material with metal fibres distributed therethrough, said corehaving a density up to about 90% of theoretical. The term metal as usedin this application is meant to include elemental metals and metalalloys.

More particularly, this invention is concerned With a method of makingthe ceramic-metal fiber body by combining a finely-divided ceramicmaterial having an average particle size not greater than 100 micronswith metal fibers having a cross-sectional area of not more than about 7to 8 square mils and a length not greater than Mr inch. The mixture ofceramic material and fiber is placed in a suitable metal tube or theannular space or spaces formed by two or more concentrically disposedtubes and packed substantially to eliminate void space, but especiallyto maintain a proper alignment of the fibers within the tube.Particularly, the fibers are positioned in a direction normal to thelongitudinal axis of the said tube.

The ends of the tube or tubes are sealed by suitable means such aswelding, and the sealed tube or tubes are swaged to a cross-sectionalarea of about 60 to of the original size.

By maintaining the reduction by swaging within the aforementionedlimits, the alignment of fibers within the element is not significantlyaltered. This stability of configuration gives license to the use ofcold working techniques contrary to a priori prediction.

Optionally, the swaged member may be sintered at an elevated temperatureto enhance bonding of the ceramic particles and/or bonding of the coreof the clad.

The ceramic material is used in a finely-divided state in order that themetal fiber can be readily distributed substantially uniformlythroughout its entire body. The average particle size is preferably notgreater than about 100 microns and it can be as small as possible, suchas, for example, about 0.1 micron. In general, the average particle sizeof the ceramic material may be from about 30 to microns. The ceramic canbe any refractory which is chemically compatible with the clad and fibermaterials at operating temperature. That is to say, no reaction shouldoccur between the metal or metals and the ceramic which would render theassembly structurally unstable. Fissionable materials in ceramic formare useful for the purpose of this invention, as well as other ceramicmaterials, such as, for example, alumina, zirconia, titania, ceria,lanthanum oxide, etc. Fissionable ceramic materials include, forexample, uranium dioxide, thorium oxide, plutonium oxide, uraniumcarbide, etc.

The ceramic material is mixed with a metal fiber which has across-sectional area of not more than about 7 square mils. The length ofthe fiber is not greater than about inch, and usually about A3 to inch.The metal fiber can be of any cross-sectional shape, namely, round,square, rectangular, or any other polygonal shape, the only limitationbeing that the cross-sectional area is not greater than about 7 squaremil-s. When the cross-sec tional area is greater than about 7 squaremils, it is found that more fiber is required to impart the same thermalcharacteristics to a ceramic-metal fiber member of a given size. Innuclear fuel elements, the use of metal fiber with an average crosssectional area greater than about 7 square mils serves to dilute thefissionable material out of proportion to gain in thermal conductivityor attainable power output.

A variety of metals can be used as clad or as metal fiber. For thermalreactors, it is desirable that the metal have a low thermal neutronabsorption cross-section. The metal has a thermal neutron cross-sectionof about 0.009 to 5.0 barns and belongs to Groups IIIA, IVB having anatomic number of at least 22 and not greater than 40, VIB or VIII of thePeriodic Chart. It is also preferred that the metal have a melting pointabove about 650 C. A variety of metals may be used for the cladding andfor the fiber, according to the specific conditions of reactoroperation. Examples of metals which are useful for this invention areAl, Fe, Ti, Zr, Ni, Mo, and alloys of these elements. If it is requiredthat the ceramic be sintered, metals structurally stable at thesintering temperature must be used for the cladding and the fiber. Somesuitable refractory metals are Mo and Cr. Fuel elements in which theceramic need not be sintered may fabricate from stainless steel, Al andNi. It is understood in all these instances that appropriate alloys mayalso be used instead of elemental metals.

The bond between the metal fiber-ceramic powder section, hereinafterreferred to as the core, and the metal clad may be either metallurgicalor mechanical in nature. If the fiber and/ or the clad may be sintered,a metallurgical bond is possible. Otherwise, the core and clad will bebonded mechanically by means of the swaging operation. Sinteringtemperatures are well known by those skilled in the art and require nofurther elaboration here except to say that in fabricating fuel elementssintering should be performed at temperatures at which dimensionalstability may be maintained.

The clad member is usually cylindrical in shape and can consist of asingle tube or at least two concentrically disposed tubes forming anannular region into which the ceramic powder-metal fiber mixture isplaced. The clad material may be of the same metal a the metal fiber orit can be a different metal. The selection of the metal for the cladmember will depend, among other things, upon the use of the finalproduct as well as whether a metallurgical or mechanical bond is desiredbetween the clad member and the core. For the fabrication of fuelelements, it is possible that zirconium fibers be used with aluminumclad; nickel or molybdenum fibers be used with stainless steel clad;stainless steel, molybdenum or aluminum fibers be used with nickel clad;or that the fiber and clad be the same metal. It is preferred, whenother considerations allow it, that the metals for the clad member andfibers be selected to procure a metallurgical bond in the final product,whereby heat transfer from the core to the clad is enhanced.

By reason of the method of preparing the final product there is no limiton the size of the cladded member which can be processed in accordancewith this invention. The clad member can be of any length, althoughusually the diameter is up to about 2 inches.

In the manufacture of the final product, the metal fibers constituteabout 5% or more by volume of the core which is placed in a clad member.The core is tamped or agitated by means of a vibratory compactor toincrease the density of the mixture. At this stage the density of themixture is about 60 to of theoretical. Compacting may be effected by anysuitable means and this means is readily known by those skilled in theart. It is understood, however, that the fibers must be aligned asbeforedescribed so as to provide a maximum of heat transfer from thecore to the clad. Plugs of the same metal as the clad member or of adifferent metal, if desired, are placed in the ends of the tube forholding the core in position. The ends of the tube are crimped or sealedby any suitable means, such as by welding. The sealed tube is thenswaged by suitable mechanical means, such as a rotary swager, to across-sectional area of about 60 to of the original value. Thi techniquemakes possible the procuremement of ceramic powder-metal fiber bodieshaving a density of from 88 to 90% theoretical. The high density impartsto the finished product unusually high thermal conductivity and highresistance to thermal stress.

Moreover, it has been demonstrated that a swaged member containing agiven amount of metal fiber possesses etter thermal conductivity thanone into which a like amount of metal powder has been incorporated.Apparently, the metal fiber, when utilized as set forth above, forms amore conductive net-work in the ceramic.

Significant and obvious financial savings may be realized in using acold working technique versus a hot pressing technique with fibrouselements.

Fuel elements may be fabricated with cores containing highconcentrations of fissionable material in ceramic form, for example, asU0 In certain instances this high concentration of fissionable materialwill permit the use of less enriched fuel. More significantly, however,is the fact that, because of the excellent thermal properties of aceramic metal fiber fuel element as described herein, greater poweroutputs may be realized for the same amount of fissionable materialpresent in the core, that is, by operation at higher reactortemperatures. By the same token, more highly enriched fissionablematerial may be utilized than otherwise practicable. It is also possibleby this technique to obtain long compacts of high density throughout thelength of the material, a result not possible by prior practices. Thistechnique eliminate the need for expensive dies and permits greaterlatitude in the size of articles which can be processed.

The only previously accepted method by which high density powder-fiberbodies can the manufactured is hot pressing at elevated temperatures,but, as already mentioned, this method of fabrication limits the sizeand shape of the final product and is too expensive for large quantityproduction. Furthermore, even when hot-pressing relatively short claddedtubular fuel elements the outer diameter cannot be closely controlledand mold-marks, such as fins, are produced. The product of thisinvention differentiates from such products by having a clad ofsubstantially uniform thickness. This difference is particularlyimportant in fuel elements where prediction of nuclear characteristicsis important. As a result, the hot-pressed element must be furtherworked to achieve proper dimensions and to remove mold-marks. Coldpressing followed by sintering results in a low density product and, ingeneral, has the inherent difiiculty that the ceramic shrinks from themetal fiber. In another fabrication method, preformed pellets areslipped into a cylindrical clad and the assembly sealed. Because thebrittleness of the core precludes swaging,

a high thermal gradient exists across the core-clad interface. Thepresent method, however, provides for a good mechanical or metallurgicalbond at the core-clad interface which results in longer life for theelement and permits use at higher temperatures.

In the event that the final product must be tubular, the techniquedescribed above is varied by placing an inner tube over a mandrel whichhas been suitably painted with graphite or other suitable lubricant. Theouter tube is placed in concentric relationship with the inner tube andspaced therefrom by means of washers or the like. As in the case of thesingle tube, the ends are sealed and the sealed cylinder is swaged inthe same way as described above.

By proper selection of the metals for the clad member and fiber, ametallurgical bond may be formed. G61 erally, the conditions forproducing such a bond involve sintering, and those skilled in the artwould understand the conditions which are necessary for this result. Inthe case of using a stainless steel clad member with either nickel,molybdenum or niobium metal fibers, sintering may be conducted at atemperature of about 1150 to 1300 C. When aluminum clad and fiber areused, sintering takes place at a temperature of about 550 to 660 C.Other examples of combinations of metals have been mentioned above. Itshould be noted that the ceramic powder may be sintered if suitablemetals are used. For example, a fuel element consisting of a molybdenumfiber-uranium dioxide core cladded with molybdenum may be completelysintered.

In order to provide a better understanding of the present invention,reference will be had to specific examples.

Example I The core comprises 90% by volume of U and 10% by volume ofstainless steel round wire, the stainless steel fiber having an averagelength of A; and a diameter of .002 inch. The U0 particles comprise 80%of 40-80 microns size and 20% from 5 to microns size. The mixture isplaced in a stainless steel tube of 0.50 inch diameter, one end of whichhas been previously crimped or sealed. The material is tamped with avibratory packer and the remaining open end sealed by welding. Beforesealing, plugs are placed at each end of the cermet to hold the cermetin place. The member is swaged by a rotary swager to a cross-section of60% the original value. The final diameter of the member is 0.35 inch.The final density is 90% Example I] The procedure in Example I isfollowed except that the original cermet contains 85% U0 and stainlesssteel. The density of the swaged member is 88%.

Example III The procedure of Example I is followed except that aluminumis used in place of stainless steel for the clad as well as the fiber.The diameter of the clad member is 1 inch. The cross-section is reducedby of the original value to give a member having a diameter of 0.78inch. The density of the swaged member is 89%.

Example I V The same method as Example HI, except that the aluminumfiber constitutes 15% by volume of the cermet. The final density is 90%.

Having thus provided a Written description along with specific examplesof my invention, no undue limitations or restrictions are to be imposedby reason thereof, the present invention being defined by the appendedclaims.

I claim:

1. In the method for fabricating a nuclear fuel element by reducing thecross-sectional area of an assembly constituted by a cylindrical coreencased in a metal sheath, said core consisting of particles of an oxideselected from the group consisting of uranium dioxide, thorium oxide,plutonium oxide and uranium oxide, said oxide particles admixed withmetal fibers present in amount equal to not less than about 5% by volumeof said core, said metal fibers having an average cross-sectional areanot greater than about 7 square mils and a length of between about inchand A1 inch, said sheath and said fibers being made of a metal selectedfrom the group consisting of aluminum, iron, titanium, Zirconium, nickeland molybdenum, the improvement comprising the steps of aligning saidfibers generally normal to the longitudinal axis of said core, andreducing the cross-sectional area of said assembly by cold swaging sameso as to effect a reduction of about 30% to 40%.

2. The improvement of claim 1 wherein said assembly is sintered afterswaging.

3. The improvement of claim 1 wherein said oxide is uranium dioxide, andsaid sheath and said fibers are made of aluminum.

4. The improvement of claim 1 wherein said oxide is uranium dioxide, andsaid sheath and said fibers are made of stainless steel.

References Cited by the Examiner UNITED STATES PATENTS 9/ 57 Handwerk etal.

OTHER REFERENCES CARL D. QUARFORTH, Primary Examiner. OSCAR R. VERTIZ,LEON D. ROSDOL, Examiners.

1. IN THE METHOD FOR FABRICATING A NUDLEAR FUEL ELEMENT BY REDUCING THE CROSS-SECTIONAL AREA OF AN ASSEMBLY CONSTITUTED BY A CYLINDRICAL CORE ENDASED IN A METAL SHEATH, SAID CORE CONSISTING OF PARTICLES OF AN OXIDE SELECTED FROM THE GROUP CONSISTING OF URANIUM DIOXIDE, THORIUM OXIDE, PLUTONIUM OXIDE AND URANIUM OXIDE, SAID OXIDE PARTICLES ADMIXED WITH METAL FIBERS PRESENT IN AMOUNT EQUAL TO NOT LESS THAN ABOUT 5% BY VOLUME OF SAID CORE, SAID METAL FIBERS HAVING AN AVERAGE CROSS-SECTIONAL AREA NOT GREATER THAN ABOUT 7 SQUARE MILS AND A ENGTH OF BETWEEN ABOUT 1/8 INCH AND 1/4 INCH, SAID SHEATH AND SAID FIBERS BEING MADE OF A METAL SELECTED FROM THE GROUP CONSISTING OF ALUMINUM, IRON, TITANIUM, ZIRCONIUM, NICKEL AND MOLYBDENUM, THE IMPROVEMENT COMPRISNG THE STEPS OF ALIGNING SAID FIBERS GENERALLY NORMAL TO THE ONGITUDINAL AXIS OF SAID CORE, AND REDUCING THE CROSS-SECTIONAL AREA OF SAID ASSEMBLY BY COLD SWAGING SAME SO AS TO EFFECT A REDUCTION OF ABOUT 30% TO 40%. 